The present invention relates to a pneumatic radial tire formed so as to improve the driving comfort of a vehicle without spoiling the steering stability thereof.
It has strongly been demanded that a pneumatic tire recently used has not only improved travelling performance of a vehicle including the steering stability thereof but also further improved driving comfort thereof. The driving comfort of a vehicle with respect to a tire thereof is represented by the magnitude of the vibration transmitted from a road surface to the interior of the vehicle via the tire. The vibration is transmitted to the interior of a vehicle via the tread of a tire, left and right side walls thereof, left and right beads thereof, a rim/disc and a vehicle body. Therefore, when the rigidity of a tire is reduced, a reaction force against an external force which the tire receives from a road surface becomes smaller, so that the driving comfort of the relative vehicle can be improved. However, when the rigidity of a tire is merely reduced, it is unavoidable that the steering stability of a vehicle with respect to the tire lowers, though the driving comfort thereof is improved.
As the techniques for driving front wheels (FWD) in a vehicle driving system and the techniques for expanding the inner space of a vehicle have been developed in recent years, the techniques for forming an asymmetric wheel structure so as to increase the inner side space thereof to as great an extent as possible have been advanced. A tendency to gradually increase the distance over which a position in which a disc is joined to a rim is offset from the center of the width of the rim toward the outer surface of the wheel has been seen. The inventors of the present invention have studied the transmission of vibration between a wheel of such an asymmetric structure and a tire to discover that there is very close correlation between the transmission of vibration and the construction of a wheel.
The results of an investigation on the transfer rates of vibration transmitted from both side walls of a tire to a wheel of an asymmetric structure show that a transfer rate of the vibration transmitted to the interior of a vehicle via a rim end on the offset side of the disc (outer side of the wheel) is higher than that of the vibration transmitted to the same via a rim end on the anti-offset side of the disc (inner side of the wheel), and that, moreover, a tendency toward a higher vibration transfer rate of the offset side portion of the wheel becomes more distinct in proportion to the distance over which the position in which the disc is joined to the rim is offset from the center of the width of the rim toward the outer surface of the wheel.
The inventors of the present invention have thoroughly studied such new knowledge with respect to the steering stability to discover that, when this knowledge is skillfully utilized, the above-mentioned problem of the steering stability and driving comfort which are in tradeoff relation can be solved, and that the driving comfort can be improved without spoiling the steering stability.